The present application is directed toward digital data communications and, more particularly, to novel methods and apparatus for transmitting digital data at a high bit rate in a power line communication system having high harmonic noise content in the power line communications media thereof.
It is often desirable to be able to communicate digital data over existing media, such as the branch circuits of the commercial power means within a building. The transmission of a modulated carrier utilizing the power line media, or power line carrier (PLC) communications, presents unique problems, because of the non-Gaussian noise voltage on the medium. This medium is typically corrupted by noise spikes occurring at intervals equal to the reciprocal of the power line frequency (60Hz. in the United States), i.e. by noise spikes which occur at 1/60 second intervals. These spikes give rise to a noise spectrum having power line frequency (60Hz.) harmonics, in addition to Gaussian "white" noise. The power line frequency harmonics decrease the signal-to-noise ratio which might otherwise be obtained, and often lead to unacceptably high error rates or a total communications breakdown. It is often not feasible to overcome the harmonic noise problem by impressing higher carrier signal voltages upon a power line communications medium. This may be due in part to legal restrictions imposed by public regulatory agencies, by adverse effect upon equipment drawing power from the branch power line, or by causing unacceptable interference levels between adjacent communication systems utilizing the same medium.
One suggested alternative for alleviating power line frequency harmonic problems has been through the use of spread spectrum techniques, in which the modulation process spreads the communication signals energy over a transmission bandwidth much larger than the original signal bandwidth. A suitable demodulator selectively sums up the signal energy present in the transmission bandwidth, and rejects the noise, whereby an improved signal-to-noise ratio is obtained. Spread spectrum modulation, and particularly modulation of the chirp frequency-modulation form, as described in Spread Spectrum Systems by Robert C. Dixon (Wiley-Interscience, 1976), has been suggested for such use. In the typical chirp spread spectrum system, relatively complex receiver and transmitter structures are required to transmit a signaling waveform changing initially in a first direction from a first frequency to a second frequency for a logic one bit and in the opposite direction from the second frequency to the first frequency for a logic zero bit. This chirp waveform presents special complications with respect to bit synchronization and additional equipment is required for providing data bit and clock synchronization at the receiving end. Additional problems may occur due to the frequency discontinuities appearing when transmitting a string of identical bits. A method for modulating a power line communication carrier to transmit high-bit-rate digital data modulated power line carrier signals and provide relatively high power line harmonic noise immunity, while providing relatively simple receiver and transmitter structures and also providing bit synchronization between system receiver and transmitter, is therefore highly desirable.